Hot water supply system for constantly maintaining temperature of hot water

ABSTRACT

The present invention relates to a hot water supply system that heats low-temperature water flowing into an inlet at a high temperature by using a heating device and supplies the high-temperature water through an outlet. The hot water supply system according to the present invention includes a heat exchanger that transmits heat of the heating device to the inflow water so as to supply the inflow water at a user set temperature; a flow sensor that measures a flow rate of the water flowing into the hot water supply system; a water tank that stores water discharged from the heat exchanger, a temperature sensor that is installed at a predetermined position on a pipe through which the water flows; and a controller that has an input unit to allow a user to input desired conditions, wherein the controller controls the operation of the heating device by comparing the user set temperature with a temperature measured by the temperature sensor and controls the operation of the heating device depending on variation in the flow rate measured by the flow sensor.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a hot water supply system for maintaining a constant temperature of hot water, and more particularly, to a hot water supply system for maintaining a constant temperature of hot water that enhances an overshoot and a undershoot of the temperature of the hot water, which temporarily occur depending on variation of usage of the hot water.

2. Description of the Related Art

In general, a hot water supply system (hereinafter, referred to as “water heater”) is used for heating and supplying water at a predetermined temperature within a short time. A performance of the water heater is primarily determined depending on how rapidly a temperature of hot water reaches a user setting temperature and whether hot water maintaining the user setting temperature can be supplied even if usage of the hot water varies.

FIG. 1 is a diagram illustrating a configuration of a known water heater having a by-pass valve and FIG. 2 is a graph illustrating a change in temperature of hot water depending on a decrease of usage of the hot water in the water heater.

The water heater includes a flow sensor 1 that senses a flow rate of inflow water, an inflow water temperature sensor 2 that measures a temperature the inflow water, a heat exchanger 3 that transmits heat of a heating device (burner) to the inflow water, a by-pass pipe 4 that directly connects an inflow pipe and an outflow pipe with each other, a by-pass valve 5 for supplying the inflow water directly to the outflow pipe through the by-pass pipe 4 without passing through the heat exchanger 3, an outflow water temperature sensor 6 that measures a temperature of hot water in the outflow pipe, and a flow control valve 7 that controls an amount of outflow hot water.

When a user decreases the usage of the hot water from a large flow rate to a small flow rate at a time P1 in the graph of FIG. 2 at the time of using the hot water, a controller (not shown) senses the decrease of the flow rate through the flow sensor 1 to reduce a heating power of the burner. However, as hot water of the large flow rate is supplied to the heat exchanger 3 before the usage of the hot water is decreased, a large amount of heat has already been supplied to the inside of the heat exchanger 3. As a result, in this state, even though the flow rate passing through the heat exchanger is decreased to thereby reduce the heating power of the burner, hot water having a temperature higher than a set temperature temporarily flows out.

In order to minimally decrease the time when the hot water having the temperature higher than the set temperature flows out (hereinafter, referred to as “overshoot”), the controller rapidly reduce the heating power of the burner for a predetermined time. Therefore, as shown in the graph of FIG. 2, the hot water having the temperature higher than the user set temperature temporarily flows out (see an arrow a) and thereafter hot water having a temperature lower than the user set temperature flows out for a predetermined time (hereinafter, referred to as “undershoot”) (see an arrow b).

As such, in order to prevent an outflow temperature from being temporarily increased when the user changes the hot water to a small flow rate from a large flow rate, the user closes the by-pass valve in normal times and opens the by-pass valve 5 to send inflow water directly to the outflow pipe without passing through the heat exchanger 3 when the overshoot occurs due to the rapid decrease of the used flow rate. The low-temperature water sent to the outflow pipe through the by-pass valve 5 is mixed with the overshot high-temperature water to thereby alleviate the overshoot of the outflow temperature.

However, in the case when the user uses the large flow rate of hot water from the small flow rate of hot water, the water heater having the by-pass valve 5 is difficult to prevent the undershoot from being increased.

In order to solve the above-mentioned problem, as shown in FIG. 3, a water heater having a flow controllable mixing valve 8 has been developed instead of the known by-pass valve.

The water heater allows a small amount of water among inflow water to flow to the outflow pipe without passing through the heat exchanger 3 by slightly opening the mixing valve 8 even in normal times. Accordingly, the low-temperature water which does not pass through the heat exchanger 3 flows into the outflow pipe and is mixed with the water passing through the heat exchanger 3, such that a control temperature of the heat exchanger is set to a temperature higher than the user set temperature. Therefore, the water passing through the mixing valve 8 is mixed with the water passing through the heat exchanger 3, such that a temperature of the hot water in the outflow pipe reaches the user set temperature.

For example, when the usage of the hot water is rapidly decreased at a time P1 shown in the graph of FIG. 4, the controller increases the amount of the water supplied directly to the outflow pipe through the by-pass pipe 4 by further opening the mixing valve 8 to thereby alleviate a temporary overshoot. Meanwhile, when the undershoot occurs due to a rapid increase of the usage of the hot water, the controller decreases the amount of the water supplied directly to the outflow pipe through the by-pass pipe 4 without passing through the heat exchanger by further closing the mixing valve 8. Accordingly, a scheme is adopted in which the set temperature is adjusted by comparatively increasing the amount of the water that flows out from the outflow pipe through the heat exchanger 3.

When the mixing valve is used as described above, the change in the temperature rapidly decreases depending on the variation in the usage of the hot water, such that hot water having a temperature comparatively close to the set temperature can be used. As shown in the graph of FIG. 4, the overshoot and undershoot of the temperature of the hot water which occurs due to the decrease in the usage of the hot water decrease in comparison with those shown in the graph of FIG. 2.

As described above, the water heater shown in FIG. 1 can enhance the temporary overshoot of the temperature of the hot water which occurs due to the decrease of the usage of the hot water, while the water heater is unable to effectively deal with the undershoot when the usage of the hot water is increased.

Contrary to this, the water heater shown in FIG. 3 can deal with both temporary overshoot and undershoot of the temperature of the hot water depending on the variation of the usage of the hot water. However, the water heater has a problem in that a temperature at an outlet of the heat exchanger is controlled to be higher than the user set temperature by approximately 20° C. on the average in order to improve controlling the temperature of the hot water depending on the variation of the flow rate when the hot water is used, such that efficiency of the water heater is decreased.

Further, as you know, a condensing water heater recovers condensed heat of exhaust gas and utilizes the heat for heating the hot water. In this case, when the outlet temperature of the heat exchanger is controlled to be high, the hot water outlet temperature is higher than a condensing temperature of the exhaust gas which is approximately 55° C. or less, such that it is difficult that the exhaust gas is condensed. Therefore, the condensed heat of the exhaust gas cannot be recovered to thereby decrease the efficiency of the water heater.

Further, depending on a quality of used water, when some of the used water contains lime, most of the lime is extracted at 55° C. or more. In the case of the water heater shown in FIG. 3, assuming that a hot water set temperature is approximately 50° C. which is a temperature generally set by the user, the heat exchanger controls the hot water set temperature to be 70° C. higher than 50° C. by approximately 20° C. so as to control the hot water outlet temperature. When water which flows into the user's inflow pipe contains the lime, the lime is extracted in the heat exchanger, such that the efficiency of the water heater is deteriorated after use over a long period and heat transmission is decreased in the heat exchanger, such that partial boiling may occur to thereby deteriorate durability of the water heater.

SUMMARY OF THE INVENTION

Accordingly, it is a first object of the present invention to provide a hot water supply system for maintaining a constant temperature of hot water that enhances an overshoot and a undershoot of the temperature of the hot water, which temporarily occur depending on variation in the usage of the hot water while maximally maintaining efficiency of a water heater.

Further, it is a second object of the present invention to provide a hot water supply system for maintaining a constant temperature of hot water that recovers condensed heat of exhaust gas to improve efficiency of a water heater by controlling a temperature at an outlet of a heat exchanger to be a condensing temperature of the exhaust gas or less in the case of a condensing water heater.

In addition, it is a third object of the present invention to provide a hot water supply system for maintaining a constant temperature of hot water that prevents lime from being extracted in a heat exchanger by controlling an internal temperature of the heat exchanger to be a lime extraction temperature or less, when inflow water contains lime components.

According to the present invention, a hot water supply system that heats low-temperature water flowing into an inlet at a high temperature by using a heating device and supplies the high-temperature water through an outlet includes a heat exchanger that transmits heat of the heating device to the inflow water so as to supply the inflow water at a user set temperature; a flow sensor that measures a flow rate of the water flowing into the hot water supply system; a water tank that stores water discharged from the heat exchanger; a temperature sensor that is installed at a predetermined position on a pipe through which the water flows; and a controller that has an input unit to allow a user to input desired conditions, wherein the controller controls operation of the heating device by comparing the user set temperature with a temperature measured by the temperature sensor and controls the operation of the heating device depending on variation in the flow rate measured by flow sensor.

Further, the hot water supply system further includes a pump that is installed on a pipe for connecting a second diverging point formed on a pipe at the side of the inlet with a first diverging point formed on a pipe at the side of the outlet, wherein when the user does not use the hot water, the controller has a preheating circulation mode in which the water in the hot water supply system is circulated through an internal circulation path for connecting the first diverging point, the second diverging point, and the heat exchanger by the pump in accordance with a preheating circulation temperature.

Further, on the basis of the set temperature which the user inputs in the input unit, the controller determines a temperature equal to or higher than the set temperature as a preheating circulation-on temperature and a temperature lower than the set temperature as a preheating circulation-off temperature and controls the heating device to start when the temperature measured by the temperature sensor is equal to or lower than the preheating circulation-on temperature and to stop when the temperature measured by the temperature sensor is equal to or higher than the preheating circulation-off temperature.

Further, when the user inputs a time in which the preheating circulation mode is performed selected from one day in the input unit, the controller correspondingly controls the preheating circulation mode.

Further, the hot water supply system further includes a check valve provided on a pipe for connecting the pump and the second diverging point.

Further, a third diverging point is formed on a pipe for connecting the first diverging point and the outlet, and a 3-way valve is provided between the first diverging point and the pump, such that preheated water in the preheating circulation mode passes through a radiator provided on an additional pipe for connecting the third diverging point and the 3-way valve.

According to the present invention, when the usage of the hot water is changed, a temporary change of the temperature of the hot water can be absorbed from a water tank, such that the hot water can be used without a rapid change in the temperature of the hot water. Further, in the case of a condensing water heater, as an outlet temperature of the heat exchanger is controlled to be a condensing temperature of exhaust gas or less, efficiency of the water heater is increased and an internal temperature of the heat exchanger can be controlled to be a temperature lower than a temperature at which lime is extracted, such that it is possible to prevent durability of the heat exchanger from being deteriorated.

Further, a water tank storing a predetermined amount of water is installed in the water heater and the water stored in the water tank and water residing in a pipe are preheated and circulated even in a state in which the hot water is not used, such that it is possible to supply hot water at a level of the set temperature within the shortest time when a user wants to use the hot water and prevent freezing of the pipe due to a decrease of an ambient temperature during the winter season.

Further, in the case of a flow path of the water during the preheating circulation process, the water passes through an outside radiator by manually or automatically operating a 3-way valve, such that the water residing in the water heater is preheated by using only one internal pump and used for a heating device.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram illustrating a configuration of a known hot water supply system having a by-pass valve;

FIG. 2 is a graph illustrating a change of a temperature of hot water depending on a decrease of usage of the hot water in the system shown in FIG. 1;

FIG. 3 is a diagram illustrating a configuration of a known hot water supply system having a mixing valve;

FIG. 4 is a graph illustrating a change of a temperature of hot water depending on a decrease of usage of the hot water in the system shown in FIG. 3;

FIG. 5 is a diagram illustrating a configuration of a hot water supply system according to a first embodiment of the present invention;

FIG. 6 is diagram illustrating a flow path of water while preheating in the hot water supply system shown in FIG. 5;

FIG. 7 is diagram illustrating a flow path of water in using hot water in the hot water supply system shown in FIG. 5;

FIG. 8 is a diagram illustrating a configuration of a hot water supply system according to a second embodiment of the present invention;

FIG. 9 is diagram illustrating a flow path of water while preheating in the hot water supply system shown in FIG. 8;

FIG. 10 is diagram illustrating a flow path of water while using hot water in the hot water supply system shown in FIG. 8;

FIG. 11 is a graph illustrating a change of a temperature of hot water depending on an increase of usage of the hot water in a hot water supply system according to the present invention; and

FIG. 12 is a graph illustrating a change of a temperature of hot water depending on a decrease of usage of the hot water in a hot water supply system according to the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The configuration and operation of preferred embodiments of the present invention will be hereinafter described in detail with reference to the accompanying drawings. Giving reference numerals to components in the drawings herein, it is noted that the same components are designated by substantially the same reference numerals, even though they are shown in different drawings.

FIG. 5 is a diagram illustrating a configuration of a water heater according to a first embodiment of the present invention. FIG. 6 is a diagram illustrating a flow path of water while preheating. FIG. 7 is a diagram illustrating a flow path of water while using hot water.

As shown in FIG. 5, the water heater 100 according to the first embodiment of the present invention includes an inflow water temperature sensor 11 that measures a temperature of water that flows into an inlet 10, a flow sensor 12 that measures flow rate of the inflow water, a heat exchanger 13 that transmits, heat of a burner to the inflow water so as to send the inflow water having a user set temperature, an outflow water temperature sensor 14 that measures a temperature of water that flows out from the heat exchanger, a water tank 15 that stores the water that flows out from the heat exchanger 13, a flow control valve 16 that controls flow rate of hot water, a pump 17 that is installed on a pipe 24 for connecting a first diverging point 21 formed on a pipe at the side of an outlet 20 with a second diverging point 22 formed on a pipe at the side of the inlet 10, a check valve 18 that prevents backflow, and a controller 30 that has an input unit 31 to allow a user to input his/her desired condition.

When the user does not use the hot water, the outlet 20 is closed and a preheating circulation mode in which the hot water circulates on a path, which is represented by a bold line in FIG. 6 is actuated by a pressure using the pump 17.

In the preheating circulation mode, the controller 30 connected with the outflow water temperature sensor 14 compares the temperature measured by the outflow water temperature sensor 14 with a set temperature which the user inputs through the input unit. For example, in the case of the user set temperature of 40° C., the controller 30 is configured to turn off heating in the heat exchanger 13 when the temperature measured by the outflow water temperature sensor 14 is higher than the user set temperature by +5° and to turn on the heating in the heat exchanger 13 when the temperature measured by the outflow water temperature sensor 14 is lower than the user set temperature by −5° C. Of course, the exemplified set temperature and permissible range may be modified by the user.

As such, by forming a closed path of internal circulation, which connects the first diverging point 21, the pump 17, the second diverging point 22, and the heat exchanger 13, the water stored in the water tank 15 and water residing in the internal pipe of the water heater 100 are preheated even when the user does not use the hot water. Accordingly, when the user starts to use the hot water, the preheated water may, all the time, reach the temperature of the hot water which is set by the user within a short time.

Further, the hot water is always circulated through the preheating circulation mode to thereby achieve an additional effect to prevent freezing the pipe of the water heater due to a decrease of an ambient temperature during the winter season.

The time when the preheating circulation is performed may be arbitrarily set by allowing the user to operate the input unit 31. For example, the preheating circulation may be always performed by the user for 24 hours. Contrary to this, the preheating circulation may be performed during a time excepting a sleeping time, for example, from 6 a.m. to 12 p.m. by the user.

When the user uses the hot water, as represented by a bold line in FIG. 7, the water that flows in through the inlet 10 is heated by the heat exchanger 13 and is discharged through the outlet 20 via the water tank 15, which is supplied to the user.

When the user opens the valve installed in the outlet 20 in order to use the hot water, water that reaches the first diverging point 21 is all discharged through the outlet 20 due to a pressure difference and as a result; the internal circulation is not performed anymore as shown in FIG. 6. The flow sensor 12 is installed on a pipe for connecting the inlet 10 and the heat exchanger 13, such that when flow of the water is sensed, a sensed signal is transmitted to the controller 30 and the controller 30 controls the temperature in the heat exchanger 13 by actuating a heating device.

Hereinafter, when the user's usage of the hot water is changed, a configuration of the present invention that is capable of minimizing a change of the temperature of the hot water by preventing temporary overshoot and undershoot of the hot water will be described.

When the user uses the hot water, the temperature of the inflow water is sensed by the inflow water temperature sensor 11 and the flow rate of the used hot water is grasped by the flow sensor 12. By this configuration, a heating power of a burner for controlling the temperature of the inflow water to reach the user set temperature of the hot water is determined and the heating power of the burner is transmitted to the inflow water by the heat exchanger 13, such that the hot water having the user set temperature can be discharged. Therefore, a temperature of the water that is discharged from the heat exchanger 13 and stored in the water tank 15 is the same as the user set temperature.

When the user increases the usage of the hot water, the increase of the usage of the hot water is grasped through the flow sensor 12. At this time, the controller 30 controls the temperature of the hot water to reach the user set temperature by increasing the heating power of the burner. However, it takes a predetermined time for the controller 30 to increase the heating power of the burner by grasping the increase of the usage of the hot water and a large amount of water flows into the heat exchanger 13 for the predetermined time. Even though the heating power of the burner is increased, the internal heat of the heat exchanger 13 applicable for the previous small flow rate gradually increases, while the flow rate of the water passing through the heat exchanger 13 increases therefor, thereby causing the undershoot in which the temperature of the water that is discharged from the heat exchanger becomes temporarily lower than the user set temperature.

However, in the case of the water heater 100 of the present invention, the water discharged from the heat exchanger 13 is mixed with the water stored in the water tank 15 before being supplied to the user. Since the hot water having a temperature close to the user set temperature is always stored in the water tank 15, the water having a comparatively lower temperature that is discharged from the heat exchanger 13 is mixed with the water stored in the water tank 15 before being supplied to the user, thereby obtaining the water having the temperature substantially close to the user set temperature.

Accordingly, since a comparatively large amount of the temporary undershoot occurring due to the increase of the usage of the hot water is absorbed in the water tank, the hot water can be supplied to the user without a remarkable change of the temperature of the hot water. Likewise, when the usage of the hot water is increased, a large amount of the overshoot which occurs by rapidly increasing the heating power of the burner in order to alleviate the undershoot may also be absorbed in the water tank.

FIG. 11 illustrates a change of the temperature of the hot water in the water heater 100 according to the present invention when the user increases the usage of the hot water. As shown in the graph, the overshoot and undershoot are remarkably decreased in comparison with the known water heater.

Even when the user decreases the usage of the hot water, the overshoot in which the temperature of the water discharged from the heat exchanger 13 temporarily increases occurs, but since the water having a comparatively high temperature that is discharged from the heat exchanger is mixed with the water stored in the water tank 15, the comparatively large amount of the overshoot is absorbed in the water tank 15. Accordingly, the amount of the overshoot is remarkably decreased, such that the hot water can be supplied to the user without the remarkable change in the temperature of the hot water. Likewise, when the usage of the hot water decreases, a comparatively large amount of the undershoot which occurs by rapidly decreasing the heating power of the burner in order to prevent the overshoot can also be absorbed in the water tank.

FIG. 12 illustrates a change in the temperature of the hot water in the water heater 100 when the user decreases the usage of the hot water. As shown in the graph, the overshoot and undershoot are remarkably decreased in comparison with the known water heater.

It is preferable that a capacity of the water tank 15 is large in order to deal with the increase and decrease of the user's usage of the hot water, but in the case in which the capacity of the water tank 15 increases, the entire size of the water heater also correspondingly increases, such that the increase of the capacity of the water tank has a predetermined limit. In general, the capacity of the water tank is in the range of 2 to 3 liters.

In the present invention, the flow control valve 14 is positioned at the side of the outlet 20, but the flow control valve 14 may be positioned at the side of the inlet 10 for convenience of design.

Hereinafter, referring to FIGS. 8 to 10, a water heater 100 according to a second embodiment of the present invention will be described.

FIG. 8 is a diagram illustrating a configuration of a water heater 100 according to a second embodiment of the present invention. FIG. 9 is a diagram illustrating a flow path of water while preheating. FIG. 10 is a diagram illustrating a flow path of water in using hot water.

As shown in FIG. 8, the water heater 100 according to the second embodiment of the present invention includes a 3-way valve 19 installed on the pipe 24 for connecting the first diverging point 21 and the second diverging point 22 in addition to the components of the water heater 100 according to the first embodiment of the present invention. Further, a third diverging point 23 is formed on a pipe for connecting the first diverging point 21 and the outlet 20 and a radiator 25 is provided on an additional pipe for connecting the third diverging point 23 and the 3-way valve 19. The 3-way valve 19 is constituted by two inlets 19 a and 19 b and one outlet 19 c to change a flow path of the water.

As shown in FIG. 9, in the case of a preheating circulation process according to the second embodiment of the present invention, when the user does not use the hot water, the first inlet 19 a of the 3-way valve 19 and the outlet 20 are closed and the second inlet 19 b and the outlet 19 c of the 3-way valve 19 are opened, such that the water internally circulates on the path shown in the figure.

By such a structure of the water heater 100, it is possible to prevent freezing of the pipe at a sub-zero temperature during the winter season and heat of the preheated and circulated hot water is transmitted to an outside radiator 25 and may be used for heating.

Another characteristic of the present invention is in that supply of the hot water and external heating can be achieved by only one pump 17 by circulating the water as shown in FIG. 9.

FIG. 10 illustrates a flow path of the water when the user uses the hot water. Even in this case, the water heater is operated in the same manner as the first embodiment of FIG. 7.

As described above, in the case of the water heater 100 of the present invention, when the usage of the hot water is changed, a temporary change in the temperature of the hot water can be absorbed through the water tank 15, the hot water can be supplied without a remarkable change in the temperature of the hot water.

Further, since a control temperature of the heat exchanger 13 is controlled to the set temperature of the hot water, a temperature at the outlet of the heat exchanger is a condensing temperature of exhaust gas or less, such that efficiency of the water heater increases and even when the water contains some lime, a lime element is not extracted, such that it is possible to prevent durability of the heat exchanger from being deteriorated.

In addition, when the user does not use the hot water, the water in the water heater is circulated through the preheating circulation mode, such that it is possible to supply hot water at a level of the set temperature within the shortest time when a user wants to use the hot water and prevent freezing of the pipe due to a decrease of an ambient temperature, during the winter season.

Further, in the case of the flow path of the water during the preheating circulation process, the water passes through the outside radiator by manually or automatically operating the 3-way valve, such that the water residing in the water heater is preheated by using only one internal pump and used for the heating device.

The present invention is not limited to the embodiment, but it will be apparent to those skilled in the art that various modification and changes may be made without departing from the scopes and spirits of the present invention.

The present invention is adopted in a water supply system, such that the present invention can supply hot water without a remarkable change in a temperature of the hot water even though usage of the hot water is changed and can be used for a heating device while preheating water residing in the water heater by using only one internal pump. 

1. A hot water supply system that heats low-temperature water flowing into an inlet at a high temperature by using a heating device and supplies the high-temperature water through an outlet, the system comprising: a heat exchanger that transmits heat of the heating device to the inflow water so as to supply the inflow water at a user set temperature; a flow sensor that measures a flow rate of the water flowing into the hot water supply system; a water tank that stores water discharged from the heat exchanger; a temperature sensor that is installed at a predetermined position on a pipe through which the water flows; and a controller that has an input unit to allow a user to input desired conditions, wherein the controller controls operation of the heating device by comparing the user set temperature with a temperature measured by the temperature sensor and controls the operation of the heating device depending on variation in the flow rate measured by flow sensor.
 2. The hot water supply system according to claim 1, the system further comprising: a pump that is installed on a pipe for connecting a second diverging point formed on a pipe at the side of the inlet with a first diverging point formed on a pipe at the side of the outlet, wherein when the user does not use the hot water, the controller has a preheating circulation mode in which the water in the hot water supply system is circulated through an internal circulation path for connecting the first diverging point, the second diverging point, and the heat exchanger by the pump in accordance with a preheating circulation temperature.
 3. The hot water supply system according to claim 2, wherein on the basis of the set temperature which the user inputs in the input unit, the controller determines a temperature equal to or higher than the set temperature as a preheating circulation-off temperature and a temperature lower than the set temperature as a preheating circulation-on temperature and controls the heating device to start when the temperature measured by the temperature sensor is equal to or lower than the preheating circulation-on temperature and to stop when the temperature measured by the temperature sensor is equal to or higher than the preheating circulation-off temperature.
 4. The hot water supply system according to claim 3, wherein when the user inputs a time in which the preheating circulation mode is performed selected from one day in the input unit, the controller correspondingly controls the preheating circulation mode.
 5. The hot water supply system according to claim 4, the system further comprising: a check valve provided on a pipe for connecting the pump and the second diverging point.
 6. The hot water supply system according to claim 2, wherein a third diverging point is formed on a pipe for connecting the first diverging point and the outlet, and a 3-way valve provided between the first diverging point and the pump, such that preheated water in the preheating circulation mode passes through a radiator provided on an additional pipe for connecting the third diverging point and the 3-way valve.
 7. The hot water supply system according to claim 3, wherein a third diverging point is formed on a pipe for connecting the first diverging point and the outlet, and a 3-way valve provided between the first diverging point and the pump, such that preheated water in the preheating circulation mode passes through a radiator provided on an additional pipe for connecting the third diverging point and the 3-way valve.
 8. The hot water supply system according to claim 4, wherein a third diverging point is formed on a pipe for connecting the first diverging point and the outlet, and a 3-way valve provided between the first diverging point and the pump, such that preheated water in the preheating circulation mode passes through a radiator provided on an additional pipe for connecting the third diverging point and the 3-way valve.
 9. The hot water supply system according to claim 5, wherein a third diverging point is formed on a pipe for connecting the first diverging point and the outlet, and a 3-way valve provided between the first diverging point and the pump, such that preheated water in the preheating circulation mode passes through a radiator provided on an additional pipe for connecting the third diverging point and the 3-way valve. 